Abstract: Provided herein are anti-idiotype antibodies that specifically recognize anti-BCMA antibody moieties, in particular, anti-BCMA antibody moieties present in recombinant receptors, including chimeric antigen receptors (CARs). The disclosure further relates to uses of anti-idiotype antibodies for specifically identifying and/or selecting cells expressing such recombinant receptors, such as anti-BCMA CAR T cells. The disclosure further relates to uses of anti-idiotype antibodies for specifically activating such cells.